In recent years, hybrid vehicles and electric vehicles are gaining attention as environmentally friendly vehicles. A hybrid vehicle has, as a source of motive power, a conventional engine and in addition thereto a motor driven by a direct current power supply via an inverter. More specifically, the engine is driven to obtain a source of motive power and the direct current power supply also provides direct current voltage which is in turn converted by the inverter to alternate current voltage employed to rotate the motor to obtain a source of motive power.
An electric vehicle is a vehicle having as a source of motive power a motor driven by a direct current power supply via an inverter.
For example, Japanese Patent Laying-Open No. 5-328529 discloses an electric vehicle having an electric motor device that is provided in a vehicular main body at a front side and generates a torque to rotate a rear drive wheel unit and configured of first and second motors exhibiting high efficiently in different operation ranges, respectively.
As described in the publication, the first and second motors have their respective rotors secured to a common, single output shaft and thus rotate around the same axis. The first and second motors generates torques, which are in turn output to the common output shaft and transmitted to a differential device via a propeller shaft coupled to the output shaft. The differential device transmits the received torques to the right and left drive wheel units, differentiating the rotations of the right and left drive wheel units.
The first motor is of a low rotation and large torque type, having a large outer diameter and a small axial dimension. The second motor is of a high rotation and small torque type, having a small outer diameter and a large axial dimension. When a required torque necessary for the electric vehicle to travel in accordance with its traveling condition(s) is calculated, an allocation of torques T1 and T2 output from the first and second motors for the required torque is determined.
Each motor's output torque allocation is determined in a method, as follows: An amount of energy supplied to the first and second motors from a power supply is calculated from the efficiency of the first motor outputting the allocated output torque T1 and that of the second motor outputting the allocated output torque T2, and output torques T1 and T2 are determined to minimize the calculated amount of energy. This allows the electric motor device's total efficiency to assume a maximum value. Furthermore, the torque required for traveling is generated by controlling driving the first and second motors, and a transmission is thus dispensed with.
Furthermore, Japanese Patent Laying-Open No. 2-133005 discloses a motored vehicle having a plurality of motors mounted therein as a driving power source. The motored vehicle determines a driving power control value for each motor to allow the motors to output driving powers, respectively, in total satisfying a driving power required for the vehicle and to also minimize the total sum of the amounts of energy provided to the motors, as based on a vehicular speed determining the mounted motors' efficiency, i.e., the motors' rotational speed, and the value of the driving power required for the vehicle.
As described therein, for example if the front wheel units and the rear wheel units are driven by separately provided two motors, and a decision is made that driving the wheel units by one motor maximizes the motors' total efficiency, then only one motor is driven. In contrast, if a decision is made that driving the wheel units by the two motors maximizes the motors' total efficiency, then the two motors are driven. In other words, a driving power required at the time is allocated between the plurality of motors to maximize the motors' total efficiency. This can increase a traveling distance per charge. Furthermore, the battery is not required to have large capacity, and the vehicle can be reduced in weight.
However, the electric vehicle disclosed in Japanese Patent Laying-Open No. 5-328529 has the first and second motors coupled to a common output shaft, and to enable the motors to exhibit high efficiency in different output ranges (or output torques and rotational speeds), respectively, providing the motors with different builds, respectively, is the only approach.
FIG. 14 shows a map in efficiency of the electric motor device described in Japanese Patent Laying-Open No. 5-328529. As can be seen from FIG. 14, the first and second motors have characteristics, respectively, as a motor, with different high efficiency ranges therein, respectively. However, such difference is uniquely determined by their difference in build, and the electric motor device as a whole has a limit to its output range that allows high efficiency. For example, in the FIG. 14 efficiency map, the first and second motors are both less efficient in a low output range, and it is considered difficult that the electric motor device as a whole maintains high efficiency until it reaches the low output range.
In the motored vehicle of Japanese Patent Laying-Open No. 2-133005, in contrast, while the two motors are coupled to separate drive wheel units, respectively, the motors are identical in build and thus have equivalent output characteristics, respectively. Thus their respective high efficiency operation ranges will generally match.
Accordingly in allocating an output torque between the two motors there is no other choice than causing the two motors to each equally output one half a required drive torque or causing one motor to output the entirety of the require drive torque. As such, an upper limit naturally arises to the motors' total efficiency that can be implemented.
The present invention has been made to solve such problem and contemplates a driving power control apparatus for a four wheel drive vehicle that can achieve high efficiency across a wide output range.